Jun 6th 2015 |

Newtonians

Above:Optical layout of a typical Newtonian

For sixty years after the invention of the astronomical telescope, all telescopes were refractors. However, refractors suffer from chromatic aberration because the different colors of light do not focus to the same point. Isaac Newton studied this problem (in his free time between inventing calculus, developing modern physics, and being beaned on the noggin by gravitationally-accelerated apples) and came to the conclusion that a refractor free of chromatic aberration could not be built. (Eventually, modern glass types have allowed excellent quality refractors to be built, but not until 300 years after Newton’s time.) Newton’s ingenious solution was to use a mirror instead of a lens to gather light. A telescope with only mirrors does not suffer from chromatic aberration. Newtonian telescopes are much less expensive to make than refractors of similar size, so they are a very popular design.

How Newtonians Work

The main purpose of all telescopes is to gather light. This is contrary to the common belief among first-time stargazers that the most important function of a telescope is to magnify objects. While telescopes do magnify objects, the most important thing they do for astronomical observing is to gather much more light than the observer’s eye alone could. Reflectors gather light by using a primary mirror located at the back of the telescope. This mirror reflects light back up the telescope tube to a small flat secondary mirror. This mirror is tilted at a 45° angle to redirect the light out the side of the telescope tube where an eyepiece is located for viewing (or a camera for photographing). The secondary mirror is supported on a spider consisting of (usually) four metal vanes. These spider vanes cause diffraction spikes which appear as the familiar cross patterns seen around bright stars in many astrophotos. While pretty in pictures, these diffraction spikes can slightly (if usually imperceptibly) degrade the image quality. Normally this is not a concern.

Newtonian telescopes produce an inverted image, making them poorly suited for terrestrial observing. For stargazing, having the image upside-down doesn’t matter because the orientation of objects in space is arbitrary. For a multi-purpose telescope for daytime and nighttime observing, a refractor or Schmidt-Cassegrain is a better choice, but for strictly stargazing, a Newtonian can be ideal.

Newtonian Mirrors

The primary mirror in a reflecting telescope is curved to focus the incoming light to a point. The simplest shape for a curved mirror is spherical. This means the reflecting surface of the mirror conforms to the shape of a sphere. The curve on a typical telescope mirror is very slight. The total depth of the curve depends on the diameter and focal length of the mirror, but typically is around a couple millimeters. The diagrams on this page greatly exaggerate the curves for clarity.

The problem with using a spherical mirror is that a spherical reflector does not focus all of the incoming light to the same point. The outer parts of the mirror will focus light to a point closer to the mirror than will the central parts of the mirror. This effect is called spherical aberration and is demonstrated in the diagram below.

In order to focus all the light to the same point, the shape of the mirror must be parabolic, as shown below.

Above:A parabolic mirror focuses off-axis light rays to a single point

Other reflectors such as satellite dishes use this same principal and are parabolic in shape. Almost all Newtonian telescopes will have parabolic primary mirrors. The only exceptions are in the case of small aperture telescopes with slow focal ratios. In such telescopes (such as the very common 4.5″ f/8 Newtonians made by just about every major telescope manufacturer) a spherical mirror is adequate because at that scale the difference between a spherical shape and parabolic shape is insignificant. For more telescopes larger than 5″ in aperture, a parabolic mirror is necessary.

Aberrations in Newtonians

While parabolic mirrors eliminate spherical aberration (which would blur the entire image), they do suffer from a less problematic aberration calledcoma. Coma is an off-axis aberration, meaning it only affects the outer parts of the image and not the center of the image. Coma results in the stars at the edge of the field having a comet-like shape with the narrow end pointed toward the center of the field.

Above:Shape of an off-axis star affected by coma. The center of the field is down in this diagram.

This effect is not too problematic for visual observation, except over very large fields of view. At high magnification, such as for planetary viewing, coma is not noticeable. Coma becomes worse for larger apertures and shorter focal ratios, so the large, fast Newtonians that might seem ideal for photography suffer the most coma and therefore require additional corrective optics to produce sharps stars across large photographic fields.

Some eyepieces provide better correction for coma in Newtonians. Standard Plössl eyepieces will show the full effect of coma in a Newtonian, but certain wide-field designs such as the TeleVue Panoptic eyepieces will provide some coma correction and show much sharper stars at the end of the field. Naturally, these are more expensive eyepieces. For most observers, coma will not be problematic. In large, short-focal-ratio Newtonians where coma is more of a problem, there are coma-corrector devices (such as TeleVue’s Paracorr) which minimize the effects of coma.

Newtonians and Dobsonians

Dobsonians are a type of Newtonian reflector. The optical design is the same as any Newtonian, but what makes a Dobsonian a Dobsonian is the type of base that it uses. Dobs have a lazy-Susan type base that allows easy movement in the up-down and left-right directions. This means Dobs are very inexpensive and very easy to use, but they do not have automatic tracking capabilities (usually) like an equatorially mounted Newtonian would. Dobsonians are covered in more detail on the Dobsonian Telescope page.

Non-Dobsonian Newtonians are normally mounted on equatorial mounts which allow automatic tracking. These are the types discussed below.

Newtonian Prices

Newtonian telescopes provide the most bang for the buck. They are the least expensive type of telescope for a given size. This makes them very popular for deep-sky observing where having a lot of aperture is important. A good-quality Newtonian reflector will start around $200-250. This will be a non-computerized scope around 4-5″ in aperture on an equatorial mount. Such a telescope will provide automatic tracking, but will not automatically locate objects. A good-quality computerized Newtonian will start around $350 and it will track as well as find objects automatically. Probably the most popular size for Newtonians is in the 6″ to 8″ aperture range. Such scopes cost around $400-600 for equatorially mounted but non-computerized models and around $800-1000 for computerized versions.

Because of their length (compared to similar-aperture Schmidt-Cassegrains, for example), larger Newtonians require very large equatorial mounts, especially if they are going to be used for photography. These scopes can be much more expensive (mostly due to the cost of the mount) and are not very portable. Large equatorial Newtonians are very uncommon now that Schmidt-Cassegrains have become popular. Equatorial Newtonians over 10″ in diameter are rare and usually cost over $2000.

Is a Newtonian Best for Me?

For beginning observers who are not interested in terrestrial observing, Newtonians provide the most scope for the money. For strictly visual observing, a Dobsonian is often the best choice since it is simpler to use and less expensive than an equatorial Newtonian. However, for observers who want automatic tracking capabilities, an equatorial Newtonian can be a good choice. However, keep in mind that 8″ and larger aperture scopes get to be quite large and cumbersome. Equatorial Newtonians can provide some photographic capabilities, but most commercial models do not have adequate mounts for such applications (although some do). For those interested in serious deep-sky astrophotography, a Schmidt-Cassegrain is often a better choice, while for visual observing only, Dobsonians are much more popular.